The regulation of purine nucleotide biosynthesis and interconversion of purine 5'-monophosphates will be studied. Multiple points of control are possible in the biosynthesis of IMP, AMP, and GMP. The production of IMP is regulated at the first committed enzymatic step, glutamine PRPP amidotransferase, while AMP and GMP levels are also maintained by regulation at the branch steps leading to their synthesis from IMP. The interconversion of IMP and AMP involves three enzymes, adenylosuccinate synthetase, adenylosuccinate lyase, and AMP-aminohydrolyase. The first two enzymes are involved in de novo synthesis of AMP. The regulation of de novo synthesis and interconversions of IMP and AMP will be evaluated in liver, muscle, brain and various tumor cell lines. The effect of anti-tumor drugs such as 6-mercaptopurine (6-MP) and its analogs will be studied in detail on the specific enzymes. We have recently shown that, contrary to previous reports, 6-MP, as the mononucleotide, is an effective inhibitor of adenylosuccinate synthetase from tumor cells (Novikoff ascites and Walker) and from liver and the inhibition is in a therapeutic concentration range. Also the adenylosuccinate synthetase from the tumor cells has unique properties and is less sensitive to feedback regulation by AMP. The altered synthetase is a specific form in the tumors and not simply caused by the rapid growth and is not a fetal isozyme. The alteration of an enzyme at a critical point in purine biosynthesis has vast ramifications in cellular regulation and offers a potential target for chemotherapy. Other objectives of this project include an expanded knowledge of multi-substrate enzyme system by study of their kinetic and regulatory properties particularly using nucleotide analogs. BIBLIOGRAPHIC REFERENCES: Laue, M.C., Yip, B.P., and Rudolph, F.B. (1977) Effects of Alanosine and Hadacidin on Enzymes Using Asparate as a Metabolite, Biochem. Pharm., in press. Clark, S.W., Berry, S.A., Ewing, R.H., an Rudolph, F.B. (1977) Regulation of Purine Metabolism: A Comparative Study of the Kinetic properties of Adenylosuccinate Synthetase from Various Sources, Comp. Biochem. and Physiol., in press.